1. Field of the Invention
Vaccination against both bacterial and viral diseases has been one of the major accomplishments of modern medicine. While effective vaccines have been developed for a large number of animal and human diseases, development of safe and effective vaccines for a number of other diseases remains problematic. In preparing suitable vaccines, the primary objectives are eliciting an immunogenic response which provides immunity against the disease of interest while assuring that the vaccine itself is non-pathogenic.
In preparing vaccines, a number of general approaches have been developed. The use of killed microbial agents as a vaccine, although generally safe, will not always be effective if the immunogenic characteristics of the agent are altered. In contrast, the preparation of live attenuated microbial agents as a vaccine will often provide improved immunologic reactivity, but will increase the risk that the vaccine itself will become pathogenic, e.g., as a result of reversion. Thus, although much experience has been gained over the years relating to the preparation of vaccines, the successful preparation of an effective vaccine against a particular infectious agent can never be assured, even when employing techniques which were previously successful for other microorganisms.
Feline viral rhinotracheitis (FVR) is species specific and enzootic in cat populations worldwide. The causative agent is a Herpesvirus (Feline Herpes I), and transmission is by direct contact or by infectious aerosols. Infection affects the nasal and ocular mucous membranes initially. Clinical signs, which commence within 2 to 10 days post infection, may include sneezing, coughing, lacrimation (excessive tearing), serous to mucopurulent nasal discharge, conjunctivitis, rhinitis, anorexia, dehydration, dyspnea, and severe depression. Cutaneous, ocular, nasal or oral ulcers and abortions may also be encountered. Pyrexia, up to 105.degree. F. (40.5.degree. C.), and a mild to moderate neutraphilic leukocytosis or mild anemia may be present. However, pyrexia and neutraphilia are mainly associated with secondary bacterial infection.
The course is often 1 to 3 weeks, but more prolonged systemic disease such as pneumonia or hepatitis may occur, especially in kittens. In fatal cases the course may extend to 4 or 5 weeks. Individual cats may die from the more severe manifestations or from secondary complications. At necropsy, respiratory tract lesions are most consistently encountered. These include hyperemic nasal and respiratory passages often covered with fibrinous or mucopurulent exudate. Secondary bacterial pneumonia may result in widely disseminated bacterial emboli. Microscopically, intranuclear inclusions, if present, occur most often in respiratory epithelium. Prior to the introduction of vaccines, 15% to 20% of isolated cat populations were reported to be asymptomatic carriers of FVR, providing a continuous reservoir for infection. Urban cat populations have carrier rates of 50% to 80%.
2. Description of the Prior Art
Feline viral rhinotracheitis was first recognized as a disease entity by Crandell and Mauer (1958) Proc. Soc. Expt. Bio. Med. 97:487-490. Experimental FVR infection results in low serum neutralizing antibody titers (e.g., 1:4 to 1:10). See, Crandall et al. (1961) J.A.V.M.A., 138:191-196; and Hoover et al. (1970) Am. J. Path. 58:269-282. Individual cats may be resistant to reinfection with FVR, although they have little or no detectable serum antibody against FVR (Bartholomew et al. (1968) Cornell Vet. 58:248-265). Infection immunity is short-lived, and cats may be reinfected six months following a primary infection. Reinfection elicits mild clinical signs and reduced viral shedding (Walton and Gillespie (1970) Cornell Vet. 60:232-239).
Previous attempts at vaccine development for FVR have included formalin inactivation (Fisher, et al. (1966) VM/SAC 61:1182-1189; Tan et al. (1971) N.Z. Vet. J. 19:12-15; and Povey et al. (1978) Feline Pract. 8:36-42); temperature sensitive mutants (Slater et al. (1976) Develop. Biol. Stand. 33:410-416); and tissue culture attenuated live virus isolates (Bittle et al. (1974) VM/SAC 69:1503-1505; Bittle et al. (1975) Am. J. Vet. Res. 36:89-91; F. Scott (1975) Feline Practice Jan.-Feb.:17-22; and Edwards et al. (1977) VM/SAC Feb:205-209). Chemically inactivated FVR vaccines failed to induce immunity(Fisher et al. (1966) VM/SAC 61:1182-1189), although later trials with formalin inactivated FVR vaccines were somewhat successful (Tan et al. (1971) N.Z. Vet. J. 19:12-15 and Povey et al. (1978) Feline Pract. 8:36-42). Formalin inactivated FVR vaccines are critically dependent on the incorporation of a suitable immunologic adjuvant such as mineral oil.
One FVR vaccine production method utilized DNA inhibitors to select biochemically uncharacterized FVR mutants that were subsequently inactivated by UV irradiation (Davis et al. (1976) VM/SAC Oct:1405-1410). Inactivation was less than 100%, and the remaining live virus was cloned at 30.degree. C. (.+-.2.degree. C.) and subjected to repeated cycles of the same process, resulting in an attenuated virus strain. See, U.S. Pat. No. 4,031,204. U.S. Pat. No. 4,287,178 discloses that temperature sensitive FVR mutants can be utilized as an attenuated live virus vaccine for FVR. Attenuated live FVR vaccines are efficacious, but may induce clinical disease or abortions. Combination vaccines have been described in which FVR and other feline pathogens are incorporated (Bittle et al. (1975) Feline Practice Nov.-Dec:13-15 and Edwards et al. (1977) Feline Practice July:45-50). These vaccines are also produced by standard procedures known to the art.
The preparation of psoralens and their use in inactivating viruses are described in U.S. Pat. Nos. 4,196,281 and 4,124,598.